1. Field of the Invention
The present invention is related to the processing and storing of holographic images and, more particularly, to a method and apparatus for dynamically storing a holographic image by oscillating between two photorefractive crystals in which interference patterns are created by four-wave mixing.
2. Description of the Related Art
Holographic images have been created, stored and manipulated in many different ways for many different purposes. Typically, a holographic image is formed using a laser, most often operating in the visible light spectrum and less often in the X-ray region or other wavelengths of electromagnetic radiation. In the following description of the prior art and the invention, the word "processing" will ordinarily be used to refer to both storage, with or without modification, and manipulation of a holographic image by, e.g., amplification, Fourier transformation, enhancement, etc. Many different types of optical elements have been used to perform such processing on laser beams carrying amplitude/phase information which will hereafter be referred to as a holographic image.
One type of optical element used to perform such processing includes a photorefractive crystal which can perform filtering and image storing functions. Such crystals may be formed of bismuth silicone oxide, barium titanate and related materials. Such crystals have been used in systems for enhancement of optical features, as taught by U.S. Pat. No. 4,674,824, to form a double phase-conjugate mirror used for image processing, interferometry and rotation sensing taught in an article by Weiss et al. in Optics Letters, Volume 12, No. 2, pages 114-116 and in a self-oscillator, as taught by Huignard et al. in Proceedings of SPIE, volume 613, pages 22-31. However, none of these systems are capable of indefinitely storing a holographic image after the original object beam discontinues to be supplied.